I joined Facebook fifteen days after it launched, becoming the five-thousand-two-hundred-and-fifty-eighth user. I remember the early Facebook well. Back then, it was still called thefacebook.com, and you had to have a Harvard e-mail address to join. You could browse profiles. You could request friendships. You could “poke” people. But you couldn’t do much of anything else. At the time, Facebook event invitations hadn’t yet been invented. Still, students browsed profiles to determine whom they wanted at their dorm-room bashes. One evening, one of my old college roommates was invited to a party by someone she had never met; he’d liked her profile picture. Inspired by his picture, she decided to go. They are now married.

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There’s another, related factor, though: the desire to broadcast the nature of these bonds. “Apes groom each other as a way of maintaining connections and making those connections public,” Sam Gosling, a psychologist at the University of Texas at Austin, told me. “That’s what Facebook does. It’s a way of publicly grooming your friends. Those conversations that happen on people’s walls could just as easily have happened in private. Facebook allows us to meet this very basic social need, and to do that on a broad scale.”